The present invention relates to a hydraulic damper adapted for use in a vehicle or the like.
As shown in FIG. 1, a prior art hydraulic damper 1 which has been in general use is so constructed that the interior of a cylinder 2 is divided into two chambers 4, 5 by a piston 3 slidably fit in the cylinder 2, the chambers 4, 5 being respectively in communication with each other through communication passages 6 formed in the piston 3. A chamber 9, defined between the cylinder 2 and an outer cylindrical body 7 and the chamber 5 in the cylinder 2 are in communication with each other through a communication passage 10 formed through a partition member 8 mounted to a bottom part of the cylinder 2. In a hydraulic damper with such a construction, a sliding movement of the piston 3 in the cylinder 2 causes the flow movement of the hydraulic fluid therein through communication passages 6. At that time, the flow of the hydraulic fluid may be controlled by damping force generation mechanisms 11, 12 comprising a plurality of valve discs and orifice passages which provide for generation of a damping force.
The damping force characteristics as obtained by the hydraulic damper 1 of the above construction are as illustrated in FIG. 2. As it is seen from FIG. 2, when the operating speed of the piston 3 is slow, the hydraulic fluid is throttled when it flows through the orifice passage, so that a representative characteristic curve is like that of a quadratic curve as designated by references a.sub.1 and b.sub.1. When the operating speed of the piston 3 is fast, the valve discs may be deflected due to hydraulic pressure, and a characteristic curve representative thereof is linear as designated by references a.sub.2 and b.sub.2.
As explained above the operation of, the prior art hydraulic damper 1 is represented by a quadratic characteristic curve with regard to the characteristics produced by the orifice passage (or orifice characteristics). Therefore at the initial stage of operation of the piston (or the interval designated by V.sub.1 in FIG. 2), a small damping force is generated out the damping force is subsequently rapidly increased.
It is to be noted, however, that a certain large damping force will be required at the initial stage of operation of the piston in order to prevent staggering of a vehicle during normal driving and to thereby provide for more stable driving. However, the above-described orifice cannot adequately meet this requirement. Further, since a large damping force will have been generated during the period following the initial operational stage (or the period V.sub.2 shown in FIG. 2), vibration caused at the side of the wheels may be transmitted to the vehicle body whereby driving comfort may be impaired.
In this way, since the hydraulic damper according to the prior art is not capable of adequately adapting the damping force provided therefore to the particular vehicle, optimum driving stability and comfort are not attained, this conventional hydraulic damper is used.